This application addresses NHGRI RFA-OD-09-004 for Medical Sequencing Discovery Projects. The ultimate goal of this proposal is to scale a new approach to identify the candidate genes and mutations that underlie rare Mendelian diseases in humans by exome resequencing. For decades, linkage analysis has been the mainstay of human genetics. However, for rare Mendelian diseases where family collection is difficult or pedigrees are small, this approach is less useful. Although the molecular bases of more than 2,600 Mendelian diseases have been determined by linkage mapping or a candidate gene approach, a nearly equal number remain to be solved (OMIM). We have assembled a collection of rare pediatric and adult Mendelian diseases that are representative of this unsolved set. In every instance, the identification of the causal gene remains intractable to either linkage mapping or exhaustive candidate gene analysis. Exome resequencing offers a new way forward for dissecting the underlying causes of rare Mendelian diseases. In our preliminary studies, we show that selective capture of protein coding sequences across the human genome coupled with massively parallel resequencing to define coding variation can accurately identify the gene underlying a monogenic disorder. In this example, comparative analysis of exome variation data from as few as two unrelated individuals affected with the disease reduced the list of candidate genes to less than ten. The candidate list was further reduced to a single gene with exome data from as few as four unrelated cases. Once identified, each candidate gene will be screened for disease-causing variants by conventional methods in a larger set of cases. Discovery of the genetic basis of a large collection of rare disorders that have, to date, been unyielding to traditional analysis will substantially expand our understanding of the biology of the human genome, facilitate accurate diagnosis and improved management of these diseases, and provide the information needed for the development of novel therapeutics. If successful, this approach is likely to replace linkage analysis as the dominant paradigm for studying diseases exhibiting Mendelian inheritance patterns and will provide a new path forward for medical genetics. PUBLIC HEALTH RELEVANCE: As we enter an era of personalized medicine, DNA sequencing will be increasingly important to public health, contributing to our understanding of the genetic basis of human disease. The targeted capture and massively parallel sequencing of all protein coding regions in the human genome (the exome) has the potential to markedly accelerate human genetics research as an efficient method for identifying highly penetrant variants at a genome-wide scale. This project will apply and evaluate exome resequencing as a new tool to rapidly identify the causes of dozens of rare genetic diseases in humans.